Aircraft manufacturers are under continual pressure to reduce the power required by commercial transport aircraft, and to improve the efficiency with which such aircraft are operated. One approach for achieving these goals has been to replace devices that historically have been driven by bleed air or shaft output from the aircraft engines, with devices that are electrically driven. For example, hydraulic pumps on newer generation aircraft are now driven by electric motors. Hydraulic pumps pressurize hydraulic fluid which is in turn used to power many aircraft systems, including landing gear, high lift devices (e.g., leading edge devices and trailing edge devices), ailerons, elevators, rudders, and/or other devices that are important for aircraft operation. Electrically-driven hydraulic pumps are expected to provide an improved measure of efficiency, flexibility, and/or reliability.
As a result of the migration to electrically-driven hydraulic pumps, the need for large electric pumps and associated electric motors and motor controllers has increased. Because the hydraulic pumps are sized to operate at conditions that the aircraft rarely encounters during normal flight, the pumps are generally operated below maximum output levels. The output of the pumps can be reduced by decreasing the speed of the pump during most flight conditions, and increasing the pump speed as demand for hydraulic pressure increases.
One drawback with the foregoing approach is that the electrically-driven pumps tend to cycle repeatedly between high and low output settings as the demand for hydraulic fluid pressure fluctuates. This can create a whining noise. Because the hydraulic pumps may be located directly beneath the passenger cabin, the whining noise can be audible and annoying to the passengers. Accordingly, there is a need to reduce the level of noise experienced by the passengers, while operating the hydraulic pumps in an efficient manner.